Deficiency in glucose-6-phosphate dehydrogenase (G6PD), an X-linked recessive red cell enzymopathy, is endemic in Southern Chinese. Universal screening of newborn is done in Hong Kong, Taiwan and Singapore, among other places. In Hong Kong, 4.8% of males are affected and seven common G6PD alleles account for over 99% of all defects. Male hemizygotes suffer from severe deficiency, while female heterozygotes may also be affected. Deficiency of G6PD may affect haematopoietic stem cell transplantation (HSCT) recipients and donors, before and after HSCT. Female patients with clonal erythropoiesis (eg myelodysplasia/myeloproliferative diseases) will have the male population incidence of G6PD. Quantitative enzyme level screening is prudent for donors and recipients, and should be repeated after engraftment. Cotrimoxazole prophylaxis should be avoided in known male and female carriers, including those with low-normal G6PD enzyme levels. Our experience suggested that G6PD-deficient marrow, stem cell and cord blood donor units have no engraftment problems. Post-engraftment G6PD levels correlate with those in donors. An acquired change in G6PD status may serve as a surrogate marker for engraftment. For female heterozygote donors with normal G6PD levels, skewing of lyonized X-chromosome ratio during engraftment may result in over-expression of the deficient allele. This can result in unexpected significant G6PD deficiency. Hence, a repeat G6PD screening at stable engraftment is recommended, especially before commencement of oxidative medications.